Marine Invertebrates: Some Can See and Some Can't

The brilliant colors of coral reef fish are matched only by those of the invertebrates that share their habitat. From soft corals to harlequin shrimps and nudibranchs, vivid colors are the rule rather than the exception. As even the colors are not simple embellishments, but a crucial part of the animal's anatomy that convey information to their own and other species.

How well can invertebrates see?

Coral reef invertebrates vary considerably in their ability to see. A great many reef invertebrates, including sponges and corals themselves, lack eyes entirely. Others, such as echinoderms, have only the most rudimentary eyes -- simple photosensitive cells that enable them to detect little more than light and dark. Many starfishes are actually attracted towards the light. Bivalves, such as clams, have eyes along the opening of their mantle cavity that allow them to detect light and movement. Both are very important when the animal is approached by a predator, such as a starfish. At the other end of the spectrum, mantis shrimps have color vision that is thought likely to be far superior to that of humans. Cephalopods, such as octopuses and cuttlefish, are also known to have excellent sight; measuring up to 16 inches (40 cm) across, the eyes of the giant squid are the largest of any animal.

Why do sightless invertebrates invest in such bright colors?

Even when animals cannot themselves see, they may still need to communicate to other animals-- particularly their predators, such as fish -- that they are poisonous. Alternatively, color patterns on such invertebrates can be used for camouflage.

Many of the most popular reef colored. As their name suggests, blood shrimps and fire shrimps are a dramatic red. Yet this helps them to blend in, because the red part of the light spectrum is quickly filtered out by water, meaning that at depths beyond a couple of feet, the shrimps appear not red but a dull green color.

What's special about mantis shrimp vision?

Although it is hard, if not impossible, to determine exactly what another animal is actually seeing, we can draw comparisons between its eye structure and our own. Human color vision is provided by three different visual pigments in our eye's cone cells. Our eye compares what it sees to these three reference points and. from this, our brain tells us what color we are seeing. In comparison, mantis shrimps have eight different visual pigments to provide their color vision -- they can see colors that we cannot even imagine! As well as this, the eyes of mantis shrimps are also well adapted for seeing how light is polarized and scattered in their environment. If that was not enough, each of their eyes is capable of moving independently and gaining a full appreciation of the visual field, especially depth perception. All these adaptations combine to produce perhaps the most amazing eyesight in the animal kingdom -- essential for such an explosively fast predator as the mantis shrimp.